This invention relates to an improved electrochemical apparatus and a method for recalibrating, in situ, an electrochemical cell of the polarographic type. The method is particularly useful in recalibrating electrochemical cells of the type disclosed in U.S. Pat. Nos. 3,260,656 issued to J. W. Ross, Jr. on Jan. 12, 1966, and No. 4,076,596 issued to J. G. Connery et al on Feb. 28, 1978.
The description of the electrochemical cells disclosed in U.S. Pat. No. 4,076,596 is hereby incorporated by reference.
As set forth in the last mentioned U.S. patent, the basic polarographic apparatus as improved by Clark is described in U.S. Pat. No. 2,013,386 and is known as the Clark cell. The Clark cell utilizes a dual electrode structure immersed in an electrolyte and encased in a membrane which is permeable to the species to be measured. Typically, when used for oxygen analysis the cathode in the Clark cell is formed of platinum or gold and is located closely adjacent to the membrane while the anode is formed of silver or lead, with an electrolyte usually made of an aqueous alkaline halide solution. In operation, the Clark cell is characterized by the fact that the cell consumes the species being measured and therefore causes a depletion of the species from the fluid sample in which the cell is immersed.
To avoid disadvantages such as stirring dependencies, a cell structure of the type shown in U.S. Pat. No. 3,260,656 was proposed by Ross. That cell utilized an electrode system which consumed the species being measured at one electrode which generated a like quantity of the species at the electrode of opposite polarity. The electrodes were closely spaced so as to avoid depletion of the species from the sample.
The Ross type cell was further improved by the Connery et al cell structure of U.S. Pat. No. 4,076,596. For the purpose of this description, those cells which characteristically deplete the sample of the species being measured are referred to as Clark-type cells, or cells which operate in the Clark mode, whereas those cells which do not deplete the sample, such as the Ross or Connery et al cells, are herein referred to generically as the Ross-type cells or cells operating in the Ross mode.
Electrochemical cells of the Ross type as described in U.S. Pat. No. 4,076,596 have been found to be very useful in applications where it is necessary to sterilize the cell before it is used in order to prevent contamination of the sample to be measured. In such applications and in others, it is not possible for the cell must be withdrawn from the sample and inserted into a known but possibly contaminated standard sample for recalibration. During the course of normal use these cells require recalibration because of the normal cell drift which takes place.
It is therefore an object of this invention to provide an electrochemical apparatus and a method for recalibrating electrochemical cells in situ. More particularly, it is an object of this invention to provide a method and apparatus for measuring the change in sensitivity of an electrochemical cell of the Ross type so that the measurement can be completed in a very short time without substantially disturbing the cell's use as a measuring element.